1. Field of the Invention
The invention relates to a method and system for measuring the intensity and phase of an ultrashort optical pulse using third-harmonic-generation (THG) based triple correlation.
2. Description of the Related Art
Ultrashort optical pulses having width of few femtoseconds has been under development over a period of time. Since electronic devices are too slow to measure temporal evolutions of ultrashort optical pulses, many techniques were developed to retrieve temporal pulse shapes.
For instance, U.S. Pat. No. 4,480,192 to Albrecht et al discloses a method of measuring an ultrashort optical pulse. According to Albrecht et al, the temporal shape of optical pulses that occur repetitively may be measured by generating an optical signal corresponding to a higher order autocorrelation function of the intensity of the pulses. The intensity of the optical mixing-signal corresponding to the autocorrelation function is detected. The optical signal is also variably attenuated to maintain constant the intensity thereof as detected. The optical pulses are measured as a function of the amount of attenuation and their temporal shape may be displayed according to the attenuation of the optical signal which corresponds to the autocorrelation function of the pulses with time.
Another approach of measuring the shape of an ultrashort optical pulse is disclosed in U.S. Pat. No. 5,754,292 to Daniel et al. According to Daniel et al, an ultrashort light pulse is split into a gate pulse and a probe pulse having a time delay with the gate pulse. Then, the gate pulse and the probe pulse are combined to form a signal pulse functionally related to a temporal slice of the gate pulse corresponding to the time delay of the probe pulse. The signal pulse is input to a wavelength selective device to output signal pulse information including intensity vs. frequency information. The time delay is varied to yield an intensity plot for signal intensity vs. wavelength and delay. The shape of the ultrashort light pulse is then extracted from the signal pulse information using an iterative algorithm.
Most of the conventional measuring techniques extract the temporal intensity either by assuming an analytic pulse shape or with the help of iterative algorithm. These techniques rely on either interference or various nonlinear effects including second harmonic-generation (SHG) and/or optical Kerr effect, and can not obtain the shape of the ultrashort optical pulse directly from the measured result.
In view of this, an approach reported by Feurer et al, xe2x80x9cMeasuring the temporal intensity of ultrashort laser pulses by triple correlation.xe2x80x9d Appl. Phys. B 66, pp. 163-168 (1998), utilizes the triple correlation of the ultrashort optical pulse to measure its intensity shape. According to Feurer et al, two stage of double-harmonic-generation and sum frequency generation are used to obtain triple correlation and thus the intensity shape of the ultrashort optical pulse. No assumptions on the pulse shape and no iterative algorithms are necessary.
However, Feurer et al do not use a process of third-harmonic-generation (THG) to obtain the intensity shape of the pulse. Furthermore, Feurer et al do not propose the method of obtaining the phase information of the ultrashort optical pulse, thus the complete knowledge of the pulse cannot be obtained.
In view of the above, an objective of the invention is to provide a method and system for measuring the intensity and phase of an ultrashort optical pulse using third-harmonic-generation based triple correlation.
Another objective of the invention is to provide a method and system for measuring the intensity shape of an ultrashort optical pulse without using an iterative algorithm.
To achieve the above objectives, the invention proposes a method for measuring an ultrashort optical pulse. A third-harmonic-generation (THG) signal with three fundamental frequency photon contributed from three different split pulses of the ultrashort optical pulse is firstly generated. The three split pulses have time delays xcfx841 and xcfx842 in between. The intensity of the THG signal is then detected while varying the time delays xcfx841 and xcfx842 between the split pulses to obtain a triple correlation of the ultrashort optical pulse. The triple correlation and its Fourier transform are used to obtain the magnitude |Ĩ(xcexd)| and the phase xcex1(xcexd) of the ultrashort optical pulse_intensity in the frequency domain, and the intensity of the ultrashort optical pulse in the time domain I(t) is determined using the magnitude |Ĩ(xcexd)| and the phase xcex1(xcexd).
According to one aspect to the invention, the spectrum of the ultrashort optical pulse may be measured for obtaining phase information. An initial phase xcex10(t) may be provided to obtain an initial guess of the ultrashort optical pulse in the time domain E(t) according to previously obtained I(t). Then, a Fourier transform is performed to E(t) to obtain E(xcexd). After reserving the phase part of E(xcexd) and_substituting |E(xcexd)| with the square root of the measured spectrum {square root over (S(xcexd))}, an inverse Fourier transform is performed to E(xcexd) to obtain the electric field information of the ultrashort optical pulse in the time domain E(xcfx84). After substituting |E(xcfx84)| with {square root over (I(t))}, an error is calculated by calculating root-mean of squared-difference between I(t) and recovered intensity |E(xcfx84)|2. The iterative algorithm is repeated and the phase is rotated when error goes to a steady value. Until the error is smaller than a threshold value, the complete information of the laser pulse can thus be obtained.
According to the invention, the intensity shape of an ultrashort optical pulse in the time domain can be obtained using third-harmonic-generation based triple correlation. The phase information can be calculated using the intensity shape obtained and an iterative algorithm of only O(n) complexity. Therefore, no assumptions on the analytic form of the pulse shape and no iterative algorithms are necessary for obtaining the intensity shape of an ultrashort optical pulse. The intensity shape can be obtained directly from the analytic solution.
The invention also provides a system for realizing the above-mentioned method. The system includes a beam splitter, a time-delay controller, a THG crystal, selecting means and a detector. The beam splitter splits the ultrashort optical pulse into three split pulses, and the time-delay controller delays two of the three split pluses to produce the time delays xcfx841 and xcfx842. The THG crystal interacts with the three split pluses, and the selecting means selects a THG signal from the THG crystal with three fundamental frequency photon contributed from three split pulses, respectively. The detector detects the intensity of the THG signal while varying the time delays xcfx841 and xcfx842 to obtain a triple correlation of the ultrashort optical pulse. As mentioned above, the triple correlation and its Fourier-transform are used to obtain the magnitude |Ĩ(xcexd)| and the phase xcex1(xcexd) of the ultrashort optical pulse in the frequency domain; and the intensity of the ultrashort optical pulse in the time domain I(t) is determined by inverse Fourier-transform of |Ĩ(xcexd)|eIxcex1(xcexd). 
The time-delay controller may include an electric-driven Galvanometer or slidable stages mounted with retro-reflector. The detector may include a spectrometer with a CCD camera, a photo multiplier tube (PMT), or a semiconductor optical detector.
Further scope of the applicability of the invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and the specific example, while indicating preferred embodiment of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.